1. Field of the Invention
The invention relates to an ultra-high intrinsic viscosity (IV) polyester resin useful in extrusion blow molding, and a method for its production.
2. Description of the Related Art
Polyester resins including resins such as poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(ethylene naphthalate) (PEN), poly(trimethylene terephthalate) (PTT), and poly(trimethylene naphthalate) (PTN), are conventionally used as resins in the manufacture of containers such as beverage bottles. Properties such as flexibility, good impact resistance, and transparency, together with good melt processability, permit polyester resins to be widely used for this application. The term resin as it is used herein includes all of the aforementioned materials.
The starting feedstocks for polyester resins are petroleum derivatives such as ethylene, which is obtained from petroleum or natural gas, and para-xylene, which is typically obtained from petroleum.
Polyester resins are generally made by a combined esterification/polycondensation reaction between monomer units of a diol (e.g., ethylene glycol (EG)) and a dicarboxylic acid (e.g., terephthalic acid (TPA)). The terms carboxylic acid and/or dicarboxylic acid, as used herein, include ester derivatives of the carboxylic acid and dicarboxylic acids. Esters of carboxylic acids and dicarboxylic acids may contain one or more C1-C6 alkyl groups (e.g., methyl, ethyl, propyl, iso-propyl, butyl, iso-butyl, tert-butyl, pentyl, hexyl and mixtures thereof) in the ester unit, for example, dimethyl terephthalate (DMT).
In conventional esterification/polycondensation processes, polyester may be formed, for example, by first producing a prepolymer of low molecular weight and low intrinsic viscosity (IV) (e.g., a mixture of oligomers), for example, by reacting a diol and a dicarboxylic acid in a melt phase reaction. The formation of the oligomers may be carried out by reacting a slurry of diol and dicarboxylic acid monomer units in an esterification reactor. EG may be lost to evaporation during the esterification reaction which may be carried out at high temperatures. Therefore the slurry of diol and dicarboxylic acid may contain an excess of EG, for example the diol and dicarboxylic acid may be present in a molar ratio of from about 1.2 to about 2.5 based on the total glycol to total di-acid. Further pre-polycondensation and polycondensation of the oligomers can be carried out to provide a resin mixture having an IV of from 0.50 to 0.65. Such resin mixtures are suitable in various applications such as fibers/filaments, fiber chips, or bottle-resin precursors. Amorphous clear base chips having an IV of from 0.50 to 0.65 may be subjected to solid-state polymerization (SSP) to increase the molecular weight (e.g., to an IV of from 0.72 to 0.76 for water bottle applications, 0.81 to 0.85 for CSD/Beer bottles, etc.). The solid-state polymerization (SSP) process unit can result in the resin undergoing crystallization which forms opaque pellets.
A continuous polyester melt-phase polycondensation process usually consists of three reaction steps: (i) esterification to form low molecular weight oligomers, (ii) pre-polymerization of the oligomers to form a pre-polymer, and (iii) polycondensation to form a polymer with an intermediate molecular weight or intrinsic viscosity (e.g., a target intrinsic viscosity of from 0.50 to 0.85).
The three reaction steps (i), (ii), and (iii) above, can be carried out to achieve the target intrinsic viscosity in from 2 to 6 reactors using existing melt-phase process technology. In general, esterification is conducted in one or two vessels to form a mixture of low molecular weight oligomers with a low degree of polymerization (e.g., about up to 5 to 10 monomer unit pairs reacted). The oligomers are then pumped to one or two pre-polymerization vessels where higher temperatures and lower pressures aid in removing water and EG. The degree of polymerization then increases to a level of 10 to 40 repeating units. The temperatures are further increased and pressures are further reduced in the final one or two vessels to form a polymer ready to be cut into pellets for example, or to be spun directly into fibers or filaments.
Esterification and pre-polymerization vessels may be agitated. Polycondensation vessels (e.g., finishers, wiped-film reactors etc.) may have agitators designed to generate very thin films. Temperatures and hold-up times are optimized for each set of vessels to minimize the degradation and other side reactions. Some by-products that may be generated by the polyester melt phase reaction include diethylene glycol (DEG), acetaldehyde, water, cyclic oligomers, carboxyl end groups, vinyl end groups, and anhydride end groups.
Both time and temperature are two variables that are preferably controlled during an esterification/polycondensation reaction. With higher reaction temperatures, the total reaction time is significantly reduced and less residence time and/or fewer reactors are needed.
Alternatively to such a continuous production method, polyesters may be prepared using a batch method. In a batch method the diol and dicarboxylic acid units are mixed together in a single reactor. In some cases more than one reactor (e.g., reaction vessel) may be used if necessary. The diol/dicarboxylic acid mixture is heated to cause the monomer units to undergo a condensation reaction. The by-products of the condensation reaction may include water or an alcohol. By conducting the reaction under reduced pressure or by subjecting the reaction mixture to reduced pressure during the final stages of the reaction, volatile by-products of the reaction can be removed thus driving the reaction to completion.
Certain physical and chemical properties of polymeric materials are negatively affected by long exposure to elevated temperature, especially if the exposure is in an oxygen-containing atmosphere or at temperatures above, for example, 250° C. Conventional methods for preparing polyester resins such as PET may suffer from disadvantages associated with the need to carry out a solid state polymerization (SSP) which subjects the resin to a long heat history and/or may require high capital expenditure.
A conventional process for producing polyester resins for container applications including melt-phase polycondensation and solid-state polymerization is shown schematically in FIG. 1 wherein the monomer components of a polyester resin such as PET are mixed in a melt-phase esterification/polycondensation reactor. The reaction is carried out to provide a molten resin having an intrinsic viscosity (IV) of from 0.50 to 0.65. The molten product obtained by the melt-phase esterification/polycondensation is then subjected to a polymer filtration.
The melt-phase esterification/polycondensation is typically carried out in a plurality of reactors. Therefore, the monomers may be added to a first esterification reactor to form a low IV material. As the oligomers pass through the remaining reactors, the IV is subsequently raised as the polycondensation reaction proceeds sequentially through a series of reactors. The material in molten form is subjected to solidification and pelletizing. The molten material may be solidified by passage of strands or filaments of the material formed by pumping the material through, for example, a die with a series of orifices. As the molten polyester resin is passed through an orifice, a continuous strand is formed. By passing the strands through water, the strands are immediately cooled to form a solid. Subsequent cutting of the strands provides pellets or chips which, in a conventional process, are then transferred to a solid-state polymerization stage (i.e., SSP).
In conventional processes for preparing polyester resins and even in some processes which avoid the use of a solid-state polymerization after polymerization is complete, the molten polymerized resin may be pumped through a die to form multiple strands. The molten resin exiting from the die is quickly quenched in water to harden the resin. As a result of the quick cooling (e.g., water quench) the molten polyester does not have time to crystallize and is solidified in an amorphous state. Solidified polyester strands, or pellets derived from cut strands, are clear, transparent and in an amorphous state.
Solid-state polymerization (SSP) is an important step in some conventional processes used to manufacture high molecular weight polyester resins for bottle, food-tray, and tire-cord applications. The clear amorphous pellets (0.50 to 0.65 IV) produced by conventional melt polycondensation reaction processes may be further polymerized in the solid state at a temperature substantially higher than the resin's glass transition temperature but below the resin's crystalline melting point. The solid state polymerization is carried out in a stream of an inert gas (usually nitrogen under continuous operation) or under a vacuum (usually in a batch rotary vacuum dryer). At an appropriate SSP temperature, the functional end groups of the polymer (e.g., PET) chains are sufficiently mobile and react with one another to further increase the molecular weight.
The SSP may include several individual reactors and/or processing stations. For example, the SSP may include a pre-crystallization step wherein the chips and/or pellets are transformed from an amorphous phase into a crystalline phase. The use of a crystalline phase polyester resin is important in later steps of the SSP because the use of amorphous polyester chips may result in clumping of the pellets since an amorphous state polyester resin may not be sufficiently resistant to adherence between pellets and/or chips. The SSP process further includes a crystallizer (e.g., crystallization step), a pre-heater, a cooler, and an SSP reactor.
One of the disadvantages encountered is that typical PET resins produced by melt polymerization have an intrinsic viscosity (IV) of around 0.50 to 0.65. When the IV is raised further by SSP, there is an initial increase in IV (known as the “lift rate”), which begins to level out around an IV of 0.90 to 1.0. Even these IV levels take a long time to achieve with conventional resins under SSP, often approaching 24 to 48 hours of SSP time. This results in excessive heat history, elevated melting temperature, and often poor color characteristics, as well as high production costs due to the energy required and slow production.
The production of a polyester resin such as PET may be carried out directly from a melt phase of the monomer units without any final solid-state polymerization. For example, a batch process may be carried out at a sufficient temperature, for a sufficient time and at a sufficient pressure to drive the polycondensation reaction to completion thus avoiding the need for any subsequent finishing (e.g., final reaction).
Some manufacturing processes do not include an SSP. Processing a polyester resin directly from a melt phase condensation to obtain pre-forms for stretch blow molding applications is described in U.S. Pat. No. 5,968,429 (incorporated herein by reference in its entirety). The polymerization is carried out without an intermediate solidification of the melt phase and permits the continuous production of molded polyester articles (e.g., pre-forms), from a continuous melt phase reaction of the starting monomers.
After pre-crystallization, the chips and/or pellets may be subjected to a final crystallization. A final crystallization may include, for example, proper heating of the chips (pellets, pastilles, granules, round particles, etc.) at appropriate temperatures. Once the polyester resin is in a crystallized state, the pellets and/or chips are preheated and ready for transfer to the top of a counter-flow SSP reactor (parallel to the pre-heater) via a pneumatic system (e.g., Buhler technology). If a tilted crystallizer is stacked above the SSP reactor, the hot/crystallized chips then enter the SSP reactor by the rotating screw of the crystallizer (e.g., Sinco technology). The SSP reactor can be considered as a moving bed of chips that move under the influence of gravity. The chips have a slow down-flow velocity of from 30 to 60 mm/minute and the nitrogen has a high up-flow velocity of about 18 m/minute. A typical mass-flow ratio of nitrogen to PET is in the range of 0.4 to 0.6. In a gravity-flow reactor, the pellets and/or chips are subjected to elevated temperatures for periods of up to 15 hours. The heating and nitrogen sweeping through the gravity-flow reactor will drive the polycondensation reaction and result in longer chain lengths and, concurrently, a higher IV of the resins.
After passing through the gravity-flow reactor, pellets and/or chips of a wide range of IV can be formed, e.g., having an average IV of about 0.80-0.84 dL/g, e.g., for CSD/Beer. The pellets and/or chips have an opaque characteristic due to their crystallinity. The crystalline material is transferred to a product silo for storage and/or packaging. The finished product in a crystalline state and having an IV of about 0.80-84 dL/g, e.g., for CSD/Beer, can be further mixed with other co-barrier resins (powders, granules, pellets, pastilles, etc.) by molders or processors who purchase the polyester resins for manufacturing, for example, bottles and/or containers.
Thus, in a conventional process, a melt-phase polycondensation process may be used to make clear amorphous pellets (typically, 0.50 to 0.65 IV) as precursors to bottle resins. The amorphous pellets are first pre-crystallized, crystallized, and/or preheated, then subjected to SSP in a gravity flow reactor (e.g., a reactor that is not agitated). After crystallization, the resin pellets become opaque and do not stick together if the temperature of SSP is at least 10° C. below the onset of the melting temperature of the resin pellets. In a direct high IV melt process as shown in FIG. 2, only the melt process (no SSP) is used to make a variety of bottle resins (e.g., 0.72 to 0.78 IV for water bottles, 0.81 to 0.87 IV for CSD/Beer bottles) as desired. In a direct high IV melt process, a finisher (e.g., a wiped- or thin-film evaporator) may be used to effectively and rapidly remove the reaction by-products such as EG (major), water, acetaldehyde, and so on. Immediate removal of EG/water under high temperatures drives the polycondensation reaction equilibrium toward the polymer side.
PET or other polyester resins are known to have hygroscopic behavior (e.g., absorb water from the atmosphere), so pellets obtained by cutting water-quenched strands contain significant quantities of water. Conventionally, the pellets may be dried by passing dry air over the pellets or by heating. Heating for an extended period at an elevated temperature may lead to problems because the amorphous polyester (e.g., PET) pellets may have a tendency to stick to one another.
Because of the challenge of achieving high IV with PET based polyesters in a cost effective, time-efficient manner, and due to the increased crystallinity that often results from the thermal history, PET resins have typically been limited to use in injection stretch blow molding to prepare products such as soda bottles or other thin wall containers. Thicker walled products, such as “handleware”, have typically been formed by extrusion blow molding (EBM) with PETG copolymer, PVC, polyethylene, or polypropylene resins, due to the ability to achieve high enough IV and melt strength with the requisite characteristics, at acceptable cost. “Handleware” is the term used for thicker walled containers (thickness of container wall being 25-55 mils) typically having handles (with handles typically having thickness of 30-40 mils), such as juice, milk, or laundry detergent bottles, and other such containers. (See FIGS. 8, 9, 10, 11 for details of a typical “handleware” container.)
There is a desire to provide PET resins that could be used to produce “handleware” through EBM, in place of PETG copolymer, PVC, polyethylene or polypropylene, with comparable or better costs and providing clearer and colorless containers (which cannot be readily produced with some other materials).